1. Field of the Invention
The present invention relates to a lens-fitted photo film unit and a sprocket wheel for the same. More particularly, the present invention relates to a sprocket wheel used with a photo film in which perforation sets are arranged and each of them includes two perforations, and a lens-fitted photo film unit incorporating the sprocket wheel.
2. Description Related to the Prior Art
There is a photo film cassette of the IX240 type, which has a cassette shell which is plastic and smaller than that of 135 type. Photo film of this type has a perforation train, which consists of a train of numerous perforation sets. Each of the perforation sets includes two perforations arranged at a small interval, and is associated with one frame. As is well-known, the IX240 type is consistent with the Advanced Photo System (trademark). Products for use with this type of photo film cassettes include a compact camera and a lens-fitted photo film unit, which is sold for example with a trade name of Fujicolor Quick Snap Super Slim (manufactured by Fuji Photo Film Co., Ltd.).
The lens-fitted photo film unit incorporates a photo film advancing device, in which a sprocket wheel is used. The sprocket wheel has two pairs of teeth, which are arranged on the peripheral face in a rotationally symmetrical manner. In the photo film, there are sections without perforations between two adjacent ones of the perforations arranged at a longer interval. There occurs a temporary state where none of the teeth on the sprocket wheel is meshed with the perforations. However one of the teeth on the sprocket wheel succeeding to a tooth having been meshed is moved to a position where it can be meshed with a succeeding one of the perforations, because the photo film advancing device has a plate spring which pushes a cam member disposed coaxially with the sprocket wheel. The cam member rotates in a direction of rotation of the sprocket wheel. The rotation of the cam member also causes a shutter to be charged and a counter wheel to indicate information of the number of remaining available frames.
The known example of the photo film advancing device has a shortcoming in that a photo film winder wheel requires much rotating force to be rotated, because the plate spring pushes the cam member forcibly. If it is conceived to reduce the biasing force of the plate spring, then the sprocket wheel can be rotated only by the reduced force. Next one of the teeth does not move sufficiently to a position of contact with the photo film. Failure is likely to occur in mesh of the sprocket wheel with the perforations. To solve this problem, there is a suggested improvement of the photo film advancing device in which a circular body of the sprocket wheel having the teeth has a relatively great radius, and a longer peripheral portion between the teeth is pressed against the photo film for the purpose of rotating the sprocket wheel with frictional force of the photo film.
In the sprocket wheel above, the frictional force considerably varies in accordance with a position of the photo film relative to the sprocket wheel. If the frictional force is low, there occurs failure in the rotation of the sprocket wheel. If the frictional force is high, it is likely that there occur scratches of the photo film or pressure fogging to the photo film.